A phase detector (phase detector) of carrier recovery circuitry (carrier recovery circuitry) has different stable points (stablepoints) Formultiple phase shift keying (Multiple Phase Shift Keying, MPSK), stable points are given on different constellation points (constellation points), where a separation between every two consecutive constellation points is 2π/M, and M is a modulation order. This is generally referred to as “2π/M phase ambiguity (phase ambiguity)”. Different de-rotate (de-rotate) logic needs to be used to correct phase ambiguity and de-rotate a symbol (symbol) to a correct constellation, and de-rotated constellation points are used for forward error correction (FEC). In a carrier tracking (carrier tracking) process or an acquisition phase (acquisition phase) process, phase estimation generally fluctuates near the aforementioned stable points. If noise introduced to a carrier restore loop exceeds a threshold, phase estimation is pushed to a neighboring stable constellation point. This effect is referred to as a cycle-slip (Cycle-Slip), and because the de-rotate logic needs to follow stable points, a cycle-slip may cause an error in FEC.
At present, a structure of a conventional receiver is shown in FIG. 1, where a received signal recovers from a signal impairment after passing through an equalizer. After being equalized by the equalizer, the signal passes through a phase estimation apparatus that estimates corresponding phase noise, removes the phase noise from the equalized signal, and then performs determining. Because a constellation diagram of a transmit signal is rotation-invariant at an angle θ relative to the origin (for example, a quadrature phase shift keying (Quadrature Phase Shift Keying, QPSK) signal and a 16 quadrature amplitude modulation (Quadrature Amplitude Modulation, QAM) signal are rotation-invariant at a 90-degree angle relative to the origin), a phase φ obtained by means of signal estimation may have a slip of θ relative to an actual phase of the received signal, that is, a cycle-slip occurs. In the prior art, training sequences are often interleaved into a received signal to avoid continuous bit errors. As shown in FIG. 2, the received signal includes several training sequence cycles (including, as shown in FIG. 2, multiple training sequence cycles such as the (k−1)th training sequence cycle, the kth training sequence cycle, the (k+1)th training sequence cycle, and the (k+2)th training sequence cycle) , and each training sequence cycle includes a data (Data) symbol and a training sequence (Pilot) symbol. The training sequence symbol is agreed by a sending side and a receiving side; therefore, if a cycle-slip occurs only in a training sequence symbol of a received signal, when a next training sequence symbol arrives, the receiving side may detect the cycle-slip, and then cut off continuous bit errors caused by the cycle-slip. However, if a cycle-slip occurs not only in a training sequence symbol, because content of a data symbol is unknown to the receiving side, a cycle-slip in this case is difficult to detect, and if the cycle-slip is not detected and corrected, burst continuous bit errors may occur from a position at which the cycle-slip occurs till arrival of a next training sequence symbol, which severely degrades performance of FEC.